database/sql: make tests repeatable with -cpu=n,n

New test added in CL 14611045 causes a deadlock when
running the tests with -cpu=n,n because the fakedb
driver always waits when opening a new connection after
running TestConnectionLeak.  Reset its state after.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/14780043

src/pkg/database/sql/fakedb_test.go

@@ -151,6 +151,8 @@ func (d *fakeDriver) Open(dsn string) (driver.Conn, error) {
 	if d.waitCh != nil {
 		d.waitingCh <- struct{}{}
 		<-d.waitCh
+		d.waitCh = nil
+		d.waitingCh = nil
 	}
 	return conn, nil
 }